Co2 decomposition via oxygen deficient ferrite electrodes using solid oxide electrolyser cell

ABSTRACT

Oxygen Deficient Ferrites (ODF) electrodes integrated with Yttria Stabilized Zirconia (YSZ) electrolyte, electrochemically decompose carbon dioxide (CO 2 ) into carbon (C)/carbon monoxide (CO) and oxygen (O 2 ) in a continuous process. The ODF electrodes can be kept active by applying a small potential bias across the electrodes. CO 2  and water (H 2 O) can also be electrolyzed simultaneously to produce syngas (H 2 +CO) and O 2  continuously that can be fed back to the oxy-fuel combustion. With this approach, CO 2  can be transformed into a valuable fuel source allowing CO 2  neutral use of the hydrocarbon fuels.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to the decomposition of carbon dioxide into carbon/carbon monoxide and oxygen via oxygen deficient ferrite (ODF) electrodes in a continuous process using solid oxide electrolyser cell (SOEC). Another application is the co-electrolysis of CO₂ and water to produce syngas for fuel or further processing. The generated O₂ can be re-circulated to the oxy-fuel combustion that will reduce fuel demand and energy requirement for the Air Separator Unit (ASU).

2. State of the Art

The attenuation of carbon-dioxide (CO₂) concentration in the atmosphere has been an important ecological issue associated with the global warming. In order to mitigate this effect, Carbon Capture and Storage (CCS), and CO₂ decomposition technologies are being developed. Currently, CO₂ is captured from flue gas by amine scrubbing or cryogenic separation. Amine scrubbing involves two steps: absorption of CO₂ at lower temperature and release the captured CO₂ to a storage unit at higher temperature [Advanced Research Projects Agency—Energy, IMPACCT 2009]. This process consumes a significant portion of the power plant energy output. Moreover, the captured CO₂ must be compressed and transported to a permanent place which is also an energy consuming process.

A preferable approach would be to decompose CO₂ into C/CO and oxygen, or co-electrolysis with H₂O to generate syngas (H₂+CO) and oxygen (O₂) [Qingxi Fu, et al. (2010), Energy Environ. Sci., 3, 1382-1397] as shown in Reaction [1] and Reaction [2].

CO₂→CO+½O₂ΔH_(600° C.) =283kJ/mole  [1]

H₂O→H₂+½ O₂ΔH_(600° C.) =247kJ/mole  [2]

Syngas and O₂ can be fed back to the oxyfuel combustion chamber that will reduce fuel demand for combustion and energy requirement for the Air Separator Unit (ASU). Syngas can also be further processed into synthetic liquid fuel (synfuel) through the Fischer-Tropsch process as shown in Reaction [3].

(2n+1)H₂+nCO→C_(n)H_((2n+2))+nH₂O  [3]

CO can be further processed into methanol by reacting with H₂ that is produced from methane (CH₄) thermal pyrolysis [Muradov et al. Catalytic Dissociation of Hydrocarbons: a Route to CO-free Hydrogen] as shown in Reaction [4] and Reaction [5].

CH₄→C+2H₂ΔH_(800° C.)92kJ/mole  [4]

CO+2H₂→CH₃OHΔH_(250° C.)=−128kJ/mole  [5]

Thus, CO₂ can be chemically transformed into a valuable energy source and its storage will not be a concern. Moreover, the generated O₂ will reduce the ASU energy requirement [McCutchen, et al. U.S. Pat. App. No. 201010146927 (published Jun. 17, 2010)].

SUMMARY OF THE INVENTION

Carbon dioxide (CO₂) is electrochemically decomposed into carbon/carbon monoxide (CO) and oxygen (O₂) by Oxygen Deficient Ferrites (ODF) electrodes. The Solid Oxide Electrolysis Cell (SOEC) consists of a thin Yttria Stabilized Zirconia (YSZ) electrolyte with ODF electrodes on both sides, working as anode and cathode. In order to keep the electrodes active, a small potential bias (<0.5V) is applied across the electrodes. CO₂ and water (H₂O) can also be electrolyzed simultaneously to produce syngas (H₂+CO) and O₂ continuously. The generated O₂ can be re-circulated to the oxy-fuel combustion that will reduce fuel demand and energy requirement for the Air Separator Unit (ASU) and thus partially offset the energy required in the decomposition process. Moreover, CO or syngas can be recovered as valuable products that can be further processed into liquid fuel through Fischer-Tropsch process. With this approach, CO₂ can be transformed into a valuable fuel source allowing CO₂ neutral use of the hydrocarbon fuels.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows the principle of ODF reactivity

FIG. 2 shows a schematic of ODF electrodes in SOEC for CO₂ decomposition into CO and O₂

FIG. 3 shows the SOEC inside NexTech Probostat™ Test Apparatus

DETAILED DESCRIPTION OF THE INVENTION

CO₂ can be actively decomposed into carbon on the oxygen-deficient ferrites (ODF) surface. The principle of ODF reactivity is shown in FIG. 1. ODF (M_(x)Fe_(3-x)O_(4-δ)) is formed by the reducing the spinal ferrites (M_(x)Fe_(3-x)O_(4-δ)) with hydrogen gas (H₂) as shown in Reaction [6]. Here M represents a bivalent metal ion such as Fe(II), Cu(II), Co(II), Mn(II), Ni(II), and so on; the oxygen deficiency (δ) expresses the degree of reduction.

Reduction H₂+O²⁻+2Fe³⁺→H₂O+Vo+2Fe²+[6]

Decomposition CO₂+2Vo+4Fe²+→C+2O²⁻+4Fe³⁺  [7]

Methanation C+2H₂→CH₄  [8]

The ODF then decomposed CO₂ into carbon as shown in Reaction [7]. In this step, carbon is deposited on the ODF surface and oxygen is transferred in the form of oxide ions (O²⁻) to be incorporated into the vacant lattice sites of ODF. This process has been demonstrated to have high efficiency (nearly 100%) to decompose CO₂ to atomic carbon at the decomposition rate of 2.9-3.5 mmol per min per gram. (Tamaura, et al., Nature 346, 255-256 (1990); Tamaura, et al., Carbon 33 (10), 1443-1447 (1995)). The deposited carbon powder can be separated by mechanical or chemical processes, or can be converted into methane or syngas. During methanation, the carbon deposited by CO₂ decomposition can be readily reacted with H₂ to form CH₄ (Tsuji, et al., Journal of Materials Science 29, 5481-5484 (1994); Tsuji, et al., Journal of Catalysis 164, 315-321 (1996)). Recently, a growing interest has been developed for electrochemical conversion of CO₂ to produce syngas and O₂ using Solid Oxide Electrolyser Cell (SOEC) [Zhan et al. Energy & Fuel 2009, 23, 3089-3096].

In the present invention, ODF electrode are integrated with YSZ electrolyte to decompose CO₂ into C/CO and O₂. FIG. 2 shows the schematic of the SOEC utilized in the present invention to decompose CO₂ electrochemically. A laboratory scale setup is also depicted in FIG. 3. The electrolyser unit cell consists of a dense electrolyte as ionic-oxygen (O²⁻) conductor and ODF-based anode and cathode electrodes. The electrolyte may be ceria-based electrolyte (eg. Gadolinium-doped Ceria (GDC or CGO), Samarium-doped Ceria (SDC)) or zirconia-based electrolyte (eg. Yttrium stabilized zirconia (YSZ), Scandium-doped zirconia (ScSZ)). ODF (e.g. nickel ferrite, copper ferrite) particles and/or several perovskite electrode materials (eg. Lanthanum strontium cobalt ferrite (LSCF), lanthanum strontium ferrite (LSF), Lanthanum strontium cobalt oxide (LSC), lanthanum strontium manganite (LSM)) combined with corresponding electrolyte materials (eg. LSCF/GDC, LSM/GDC, ODF/GDC, LSM/YSZ) serve as electrodes for anode and cathode respectively. Analogous to the fuel cell technology, the proposed setup can be easily scaled up.

A feed which may contain CO₂ or CO₂+H₂O flows from a feed source 1 through the cathode side channel 2 and react with the ODF electrode 3. A small potential bias is applied from the external source 4 that keep the ODF electrodes active. The electrode decomposes CO₂ into CO and oxide ions O²⁻as shown in Reaction [9].

CO₂+2e⁻→CO+O²⁻  [9]

O²⁻→½O₂+2e⁻  [10]

The generated oxide ions migrate thorough the YSZ electrolyte 5 to the anode electrode 6 and thus complete the cell internal circuit. At the anode electrode the oxide ions combine to generate oxygen and shown in Reaction [10], which flow through the anode side channel 7.

EXAMPLE

A preliminary test was performed according to the embodiments of the invention to establish the feasibility of the inventive process. The test set is shown in FIG. 3. A button cell 8 manufactured according to the description in FIG. 2. The button cell was mounted inside the NexTech Probostat™ 9 button cell test apparatus using AREMCO-516 high temperature cement. Alicat™ mass flow controllers (MFCs) were used to control the flow rates, pressure and compositions. Concurrently, the electrochemical performances were measured using Reference 300™ Potentiostat/Galvanostat/ZRA (Gamry Instruments, Warminster, Pa.) 10.

The button cell was heated from room temperature to 750° C. at a rate of 1° C./min. During this period, the anode and cathode were exposed to 50 sccm of N₂. After that, 100 sccm H₂ was provided to anode and cathode side, respectively, to reduce NiFe₂O₄ into ODF at 750° C. Once the reduction of electrodes was completed, the cathode was supplied with 60 sccm of CO₂. The experiment investigation was carried out at 750° C. and the cell electrochemical performances were measured using Reference 300 Potentiostat/Galvanostat/ZRA (Gamry Instruments, Warminster, Pa.), and exhaust gases were analyzed via Gas Chromatography (GC) 11. This test confirmed the feasibility of CO₂ electrolysis via ODF electrodes in a continuous process as shown in Table 1.

TABLE 1 Gas Chromatography Analysis Description After After After After decomposition decomposition decomposition decomposition for 6 hrs for 150 hrs for 461 hrs for 531 hrs Cathode Anode Cathode Anode Cathode Anode Cathode Anode Side Side Side Side Side Side Side Side Compound (%) (%) (%) (%) (%) (%) (%) (%) CO₂ 44.72  3.04 49.48 5.06 47.23 3.41 44.20 5.05 CO ND ND ND ND 1.16 ND 0.52 0.14 O₂ 13.35 79.32 13.16 94.16  14.05 56.08 13.94 84.48 H₂ ND ND ND ND ND ND ND ND Ar  8.17 ND  8.45 ND 4.84 8.94 3.79 9.55 N₂ 33.76 17.64 28.91 0.78 32.72 31.58 37.34 0.77 

1. A method to decompose CO₂ into C/CO and O₂ using Oxygen Deficient Ferrites (M_(x)Fe_(3-x)O_(4-δ), M represents a bivalent metal ion such as Fe(II), Cu(II), Co(II), Mn(II), Ni(II), and so on) electrodes integrated with solid oxide electrolyser cell. 